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Science: Fourth neutrino: theory leaves cosmology intact

7 September 1991

By MARCUS CHOWN

A ‘fourth’ neutrino can exist without upsetting physics and astronomy,
according to an American physicist. Myron Bander of the University of California,
Irvine, has come up with a plausible mechanism for giving such a neutrino
a mass and also a short decay time, so that it does not have too great an
effect on the evolution of the Universe. The decay of Bander’s neutrino
involves no hypothetical particles other than those already postulated by
particle physicists for other reasons (Physical Review Letters, vol 67,
p 801).

In the past few years, several groups of physicists have obtained tentative
evidence for the existence of a fourth neutrino, which has a mass. The neutrino,
weighing in at 17 kiloelectronvolts (keV), seems to make an appearance in
about 1 per cent of all normal beta decays of atomic nuclei (New Scientist,
Science, 2 February).

The problem has always been that laboratory experiments, together with
considerations of astrophysics and cosmology, severely constrain the properties
such a heavy neutrino can possess. In particular, the particle must decay
in less than 1014 seconds. If this were not the case, the gravity of all
the heavy neutrinos in existence would long ago have halted the Universe’s
expansion and made it re-collapse. The formation of elements in the big
bang and Supernova 1987A also constrain the properties of the particle.

According to Bander, the new particle is a ‘Dirac neutrino’ with a lifetime
of about 1012 seconds – which is good news for cosmology. It decays into
an ordinary neutrino and a hypothetical particle known as an axion. Although
no one has ever observed an axion, the particle is required to ‘break the
symmetry’ in the so-called supersymmetric theories, which attempt to unify
gravity with the other three forces of nature.

Bander’s is not the only explanation for the 17 keV neutrino. The last
came from two Japanese physicists, who claimed that their theory had the
advantage that it fitted in well with the favoured explanation for the shortage
neutrinos from the Sun (New Scientist, Science, 13 July).